[koolba]

18 hours?! I’ve never heard of that for making yogurt. Is that a low temp pasteurization?

I’ve only done the usual 180F, followed by waiting for it to cool, finally add the starter and mix.

[BeetleB]

When I try to use UHT milk, I usually fail in making yogurt. But that may be because I do 6.5 hours and he's doing 18 hours. Perhaps you need to go so long with UHT?

[Glawen]

Well he probably uses a machine (yaourtière in French) which maintains the pots at 40 degrees. My parents use this with UHT to do yoghourt.